The invention relates to a method of scrambling, at the level of a transmitter, and descrambling, at the level of receivers, a specific television broadcast of limited duration, scrambling being performed by means of a pseudo-random generator which is initialized by a control word, descrambling involving the reconstruction upon reception, by means of a memory card device, of the control word which initializes a pseudo-random generator which is identical to that of the transmitter, said pseudo-random generator supplying the data necessary for descrambling. The invention also relates to a transmitter device for use in carrying out the method in accordance with the invention, said transmitter device comprising a pseudo-random generator which is initialized by a control word, and to a receiving device for use in carrying out the method in accordance with the invention, said receiving device comprising a pseudo-random generator which is initialized by a control word, and comprising a memory card reader. This method and these devices are used in scrambled television transmission and reception systems.
A television scrambling method of this kind is known from GB-P-2 132 860 (British Broadcasting Corp.). In this satellite television system, the control word is formed in a fully random manner and is transmitted in encrypted form to the receivers. The latter decrypt the control word by way of a calculation which is performed in an "smart" card by a process which is known per se, in this case being the so-called "DES" process with a secret key. Moreover, each card has a different identification code.